Numerous applications exist in industry where it is desirable and necessary in some cases to provide protective and/or aesthetic features to a surface. Such applications include use of a paintable sealer for automobile bodies. Historically, a variety of materials have been used as sealers to fill voids in structures and exclude dirt, moisture, and other materials.
Sealers have been supplied as liquids or solids depending upon the demands of the application. In the automotive industry, paste-like plastisols have been used for decades to seal metal seams, as described in U.S. Pat. No. 4,900,771 (Gerace et al.). These materials function by having PVC (polyvinyl chloride) particles swell in a plasticizer when heated, and fuse into a solid material. Typically, paint adheres poorly to PVC based sealers due to the high levels of plasticizer. In addition, PVC sealers cannot be recycled, and when burned, give off HCl. For this reason they are not used in Europe.
Hot melt sealants and adhesives are generally solid thermoplastic materials which quickly melt with heating and then form a firm bond on cooling. A typical class of hot melt adhesive compositions utilizes polyolefins which are known in the art to be difficult to paint and which have poor adhesion to non-porous metallic surfaces, such as steel and aluminum.
In use, a bead of the liquid sealer is applied on the joint seam, in the way caulking is applied, and the worker must brush or level the material out into a relatively uniform film. The application of a liquid sealer takes skill and often results in a poorly sealed seam. Liquid sealers cannot be used for visible applications due to non-uniform appearance.
Recently there has been a trend towards more user-friendly sealer systems such as ropes or tapes because the handling properties of these materials make for fast installation and eliminate the need to finesse the material after application. Tapes and ropes of PVC-based sealant material have begun to find niche applications. Other materials have also been supplied as a strip or tape.
U.S. Pat. No. 3,659,896 (Smith et al.) describes a semi-cured, curable polymeric sealing strip composition based on a liquid polysulfide polymer, for adhering and sealing a windshield to an automobile body. The sealing strip has adhesion to both the glass and metal such that the windshield is immediately sealed at room temperature; further cure of the sealant material occurs on exposure to moisture at ambient conditions.
U.S. Pat. No. 4,490,424 (Gerace) describes a hot-melt adhesive and sealant tape in which the tape comprises a core of hot-melt adhesive encased in a sheath of plastic resin. The plastic resin is compatible with the hot-melt adhesive core in both liquid and solid states.
A need exists in industry for a user-friendly, paintable, meltable sealant material that can be used for visible and non-visible applications and handled as a strip or tape.
Thermosettable pressure-sensitive adhesives are known and have utility in a number of industries including assembly of automobiles and appliances. Such adhesives are described in U.S. Pat. No. 5,086,088 (Kitano et al.). These adhesives are pressure-sensitive, i.e., tacky at the temperature of bonding, and are typically used in the form of a pressure-sensitive adhesive transfer tape in which the layer of adhesive is provided on a release liner. The transfer tape can further include a nonwoven web for reinforcement of the adhesive layer. In use, the transfer tape bonds one surface to another surface at ambient temperature. The surfaces are then heated to a temperature sufficient to cure the adhesive to a thermoset state.
In some applications it would be desirable to have a thermosettable pressure-sensitive adhesive tape that has a non-tacky surface that can be activated to an adhesive state at the temperature of use.
One such application is in some automotive assembly lines where the doors are temporarily attached to the vehicle body by bolting the hinges on to the body prior to painting. The door is positioned on the vehicle by aligning the door hinges on slotted holes in the car body, and then fastening the hinges to the body with one or more washers and corresponding bolts. After the vehicle body has been painted, the doors are removed from the hinges so that interior parts can be installed. It would be desirable to have the washers fixed in place on the hinges so that when the doors are re-attached, they will be precisely aligned without having to take time to re-align them.
Japanese Patent Publication (Kokai) No. 64-67417 describes a washer fixed to a door hinge with a tacky thermosetting adhesive film. The washer serves as an alignment member for a bolt that is used to join the hinge to a door. The film is tacky on both sides and is prone to contamination from dust, oil, etc., which can be found in assembly plants. The contaminated surface, in turn, must be cleaned to ensure an adequate bond. The film also tends to be very thin so that it can be difficult to handle, and removing the liners so that the film can be bonded to the washers and the bolted surfaces can be a labor intensive operation which prohibits automation of the assembly line.
It is known to saturate a nonwoven fabric as a support with a thermosettable adhesive to increase the rigidity of the adhesive so that it can be handled more easily, but this would add cost and does not get around the other deficiencies of the above-described adhesive film.
Japanese Patent Publication (Kokai) No. 53-42280 describes a composite sheet having a sheet of thermosetting material that is coated with a heat fusing material. The heat fusing material is intended to coat the thermosetting resin sheet so that workers can avoid direct skin contact with the thermosetting adhesive. The thermosetting material and the heat fusing material are mutually non-reactive and compatible, and characterized by a maximum difference in fusing temperatures of 50.degree. C. The heat fusing material melts and mixes with the thermosetting material before it is hardened.
Japanese Laid-Open Patent Application JP H4-189885 describes a thermosettable pressure-sensitive adhesive made from acrylate copolymers and epoxy resin. The adhesive composition can be coated onto one or both sides of a nonwoven material, which acts as a pre-preg to increase the strength of the adhesive sheet.
It would be desirable to have a thermosettable pressure-sensitive adhesive tape that is substantially tack-free at room temperature (about 21.degree. C.) on at least one major surface, but both major surfaces of the tape can be adapted for bonding to other substrates.